1. Field of the Invention
This invention relates to modified thermoplastic linear polyester compositions which are useful as molding compositions. More particularly, this invention relates to modified polyester based molding compositions which comprises an effective amount of sodium citrate which functions to facilitate the formation of a shaped, molded article having improved surface characteristics by increasing the crystallization temperature of the polyester.
2. Description of the Prior Art
Thermoplastic polyesters, such as poly(ethylene terephthalate), and poly(butylene terephthalate), can be produced on a large scale at relatively low cost, and have found extensive use in the production of synthetic fibers. However, unmodified polyesters have not been exploited as molding resins because of certain inherent disadvantages, as for example insufficient toughness for that utility; and a lack of dimensional stability when heated to elevated temperatures.
Accordingly, for various applications, it is now desirable to modify or improve the molding properties of various polyesters, and for this purpose various additives have been incorporated into polyesters. For example, the strength, dimensional stability and hardness of polyesters can be enhanced by incorporating reinforcing materials, such as glass fibers, asbestos fibers, or other fibrous materials.
However, while effective to a limited extent, incorporation of such fibrous materials provide certain other disadvantages. For example, when employing such fibrous strenghtening materials, it is necessary to use very high mold temperatures, i.e., temperatures in the range of from about 130.degree. C. to about 180.degree. C., to obtain good release of the molded article from the mold, and to obtain molded articles which possess a glossy surface appearance, and surfaces which are not rough in texture. While good surface characteristics, i.e., high gloss, and smooth texture, can be obtained at molding temperatures of about 130.degree. C. or more, the use of such high temperatures are not practicable in the molding field, because most molds are heated with water, and can obtain molding temperatures of only from about 77.degree. C. to about 110.degree. C. A few molding devices which employ heating means such as oil can attain higher molding temperatures, however, these devices are generally inconvenient and hard to use, and still either do not attain the desired higher temperatures, or attain them unevenly because of inadequate coring.
Because of these heating difficulties, it is desirable and there is a need for a polyester based molding composition capable of being molded at lower temperatures i.e., from about 85.degree. C. to about 100.degree. C.; yet which still provides a molded product having good physical properties such as high flexural strength, modulus, tensile strength and impact strength, good finished appearance and mold release characteristics, and high melt flow index for sufficient flow into the mole. In order to accomplish these goals, it is necessary for crystallization of the polyester to begin at as high a temperature as possible upon cooling of the polyester in the mold and that crystallization of the amorphous polymer began at as low a temperature as possible. Various prior art references disclose that nucleating agents can be used with a crystallizable polyester, as for example linear saturated polyesters of aliphatic diols and aromatic carboxylic acids, so that the polyester can be molded at mold tempertures of below about 110.degree. C. and still provide a molded article having a smooth and glossy surface. For example, U.S. Pat. Nos. 3,435,093; 3,516,957; and 3,639,527; and Netherlands Patents Nos. NL 79-01605 and NL 79-01609, generally disclose the use of co-polymers of .alpha.,.beta.-unsaturated olefins and .alpha.,.beta.-unsaturated carboxylic acid, as for example, ethylene/methacrylic acid copolymer, in which all or portion of the pendant carboxyl functions have been neutralized forming the alkali metal salt of the copolymer, as nucleating agents for the aforesaid purpose. While relatively effective, there are various disadvantages associated with the use of these prior art nucleating agents. For example, such agents provide a reduced melt index making it harder to fill thin cross-sections in complicated shapes.